Rotary vacuum filters were introduced shortly after the turn of the century to separate minerals from dirt and water in the mining industry. A drum was rotated in a vat in which the minerals and water were suspended. A vacuum was created in a downleg of the rotary vacuum filter as clean shower water was sprayed on the surface of the rotating drum. The waste water passed through the drum under force of the vacuum leaving minerals entrapped on the drum surface.
Rotary vacuum filters were subsequently utilized in the pulp and paper industry to separate pulp from the cooking or process liquids. Rotary vacuum filters used to process minerals had no problem with atmospheric spraying of the wash water, because the liquid displaced in mineral washing was not viscous. Pulp, however, is cooked with a liquor that forms a gluey viscous substance referred to as black liquor that must be removed from the pulp in brownstock washing. Any liquor not removed by the brownstock washing is carried on through the process to the deckers, savealls and bleach plant.
For years the rotary vacuum filters used in brownstock washing have been environmentally inefficient in their use of fresh water and exhaust emissions. Conventional methods of spraying shower water in a substantially open atmosphere, such as whistle, spoon, fluid flow and weir showers, cause air to become entrapped in the black liquor. In conventional systems, as much as fifty percent of the filtrate passing through the downleg consists of air molecules. The entrapped air restricts the black liquor drainage rate through the filter drum and downleg thereby reducing the efficiency of the system. In addition, entrapped air causes a tremendous amount of foaming throughout the brownstock system. Accordingly, pulp washing systems have required the use of expensive filtrate tanks and defoamers to allow the entrapped air to migrate to the surface of the black liquor tank prior to being pumped to evaporator storage. The overall impact of air entrapment is a reduction in washing efficiency that results in more fresh water being required to clean a pound of paper pulp.
In view of the above, it is an object of the present invention to provide a sealed shower system for a rotary vacuum filter that overcomes the air entrapment problems associated with the conventional brownstock and bleach washers.
It is a further object of the present invention to provide a seeded shower system for a rotary vacuum filter that is incorporated into a low profile hood to reduce exhaust emissions of a rotary vacuum filter.